The invention proceeds from a fluorescent lamp in accordance with the preamble of claim 1.
In the case of the known fluorescent lamps with outside tube diameters of greater than 26 mm, such as the T12 lamps with 38 mm, for example, the temperature of the cold spot at which the excess mercury condenses and which ensures an optimum luminous flux during operation of the lamp, is designed for an ambient temperature of approximately 25xc2x0 C. The cold spot is situated in this case in the middle of the discharge tube on the inner glass wall.
In the case of the new fluorescent lamps with outside tube diameters of less than or equal to 26 mm (T8, T5), which have been developed specifically for interior lighting (ambient temperature of greater than 25xc2x0 C.), the temperature of the cold spot must be approximately 40xc2x0 C. so that an optimum light yield is achieved. This is reached at an ambient temperature of approximately 35xc2x0 C. With decreasing inside diameter, it is necessary to displace the cold spot from the middle of the lamp to a point behind the electrodes, since because of the ever increasing current density it is no longer possible to reach such a low temperature of approximately 40xc2x0 C. in the middle of the discharge tube. For this purpose, one electrode is displaced further into the discharge tube by the formation of a longer stem seal, so that a cold spot of approximately 40xc2x0 C. can form behind this electrode.
Since these lamps have been very well received because of their high light yield and their slim finish, it was of interest also to make use of these lamps in exterior lighting (ambient temperatures of less than 25xc2x0 C.). However, the temperature of the cold spot behind the electrode is not designed for this purpose, with the result that it is not possible to achieve optimum light yields with these lamps.
It is the object of the present invention to provide a fluorescent lamp having a discharge tube with an outside diameter of less than or equal to 26 mm in accordance with the preamble of claim 1, which ensures an optimum light yield even in the case of exterior operation with relatively cold ambient temperatures of less than 25xc2x0 C. It should be possible to achieve the object using simple means, and there should be no fundamental changes to the lamp design as a result.
This object is achieved by means of the characterizing features of claim 1. Particularly advantageous refinements are to be found in the dependent claims.
By providing a material which is a good conductor of heat in the region of the electrode displaced further into the discharge tube, the heat in the glass tube in the region of this electrode can be directed toward the base. As a result, the cold spot is displaced from a point behind the electrode into the middle of the lamp again, where it then acquires an optimum temperature because of the lower outside temperatures of less than 25xc2x0 C.
The thermally conducting material preferably consists of a thermally conducting coating on the outer wall of the discharge tube, or of a thermally conducting foil which surrounds the discharge tube in this region over the entire outer circumference. Such a thermally conducting material is also preferably applied or provided outside on the glass tube in the region of the other sealed electrode.
For optimum heating, the thermally conducting material should reach at least from the electrode filament to the edge of the base shell. It is even better if the thermally conducting material reaches beyond the electrode filament in the direction of the middle of the glass tube andxe2x80x94in the case of a foilxe2x80x94is soldered or welded to the case shell. In the case of a thermally conducting foil, the latter preferably contains aluminum or copper.